Multi-piece solid golf ball

ABSTRACT

A golf ball is provided that achieves a satisfactory distance on full shots with an iron, is superior in the short game, and has a good feel at impact and a good durability. The golf ball has a core formed of a rubber composition as one or more layer, an envelope layer formed of a resin material as one or more layer, an intermediate layer formed of a resin material as one layer, and a cover formed of a resin material as one layer having a thickness of not more than 1.0 mm. The layers of the ball have Shore C hardnesses at the respective surfaces thereof which together satisfy certain specific conditions, the core and the ball have respective deflections which satisfy certain conditions and the core has a specific internal hardness profile.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2021-191634 filed in Japan on Nov. 26, 2021, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a multi-piece solid golf ball of four or more layers that has a core, one or more envelope layer, an intermediate layer and a cover.

BACKGROUND ART

Many innovations have been made in designing golf balls with a multilayer construction, and numerous balls that satisfy the needs of not only professional golfers, but also high- and mid-level amateur golfers, have been developed to date. For example, functional multi-piece solid golf balls in which the surface hardnesses of the respective layers—i.e., the core, envelope layer, intermediate layer and cover (outermost layer)—have been optimized are in wide use. Also, a number of technical disclosures have been published which focus on the hardness profile of the core that accounts for most of the ball volume and which, by creating various core interior hardness designs, provide high-performance golf balls for professionals and mid- to high-level amateur golfers.

Examples of such literature include JP-A H9-248351, JP-A 2006-326301, JP-A 2007-319667, JP-A 2012-071163, JP-A 2007-330789, JP-A 2008-068077, JP-A 2009-034507, JP-A 2009-095364, JP-A 2016-101254, JP-A 2016-116627, JP-A 2009-095358, JP-A 2016-101256, JP-A 2008-149131, JP-A 2009-095365, JP-A 2009-095369, JP-A 2020-089633 and JP-A 2021-037157. These disclosures, all of which relate to golf balls having a multilayer construction of four or more layers, focus on, for example, the surface hardnesses and thicknesses of the respective layers—namely, the core, envelope layer, intermediate layer and cover (outermost layer)—and the core hardness profile.

However, there remains room for improvement in optimizing the hardness profile of the core and the thickness relationship among the layers in these prior-art golf balls. That is, these golf balls, even if they are able to retain a good distance on driver (W #1) shots, often fall short in terms of their distance on iron shots. Moreover, with some of these prior-art golf balls, when an attempt is made to achieve a superior distance performance not only on driver shots but also on iron shots, a sufficiently high spin rate on approach shots cannot be achieved, resulting in a ball that lacks a high playability or that has a less than satisfactory feel on full shots.

Also, for a certain level of player among amateur golfers, there is a possibility that a distance performance on iron shots which is superior to the distance performance on driver shots can improve the golf playability. Hence, there exists a desire for the development of a golf ball which, at this level of player, has a greatly improved distance performance on iron shots and also has a high playability in the short game while exhibiting other good properties such as feel and durability.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a multi-piece solid golf ball which has a good controllability in the short game while ensuring an increased distance on iron shots, has a soft feel at impact and exhibits a good durability to cracking on repeated impact.

As a result of extensive investigations, I have found that the above object can be achieved in golf balls having a core, an envelope layer, an intermediate layer and a cover by forming the cover so as to be relatively soft, forming the intermediate layer so as to be relatively hard, and also forming the envelope layer adjoining the inner side of the intermediate layer as one or more layer that is softer than the intermediate layer and harder than the core surface. That is, I have discovered that remarkable effects can be achieved by forming the core of a rubber composition as one or more layer, forming the envelope layer of a resin material as one or more layer, forming the intermediate layer of a resin material as one layer and forming the cover of a resin material as one layer having a thickness of not more than 1.0 mm; by setting the hardness relationship among the layers such that the surface hardness of the core, the surface hardness of the sphere obtained by encasing the core with the envelope layer (envelope layer-encased sphere), the surface hardness of the sphere obtained by encasing the envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) and the surface hardness of the ball satisfy the conditions (surface hardness of envelope layer-encased sphere)>(surface hardness of core) and (surface hardness of intermediate layer-encased sphere)>(surface hardness of ball); and moreover by designing the golf ball such that, letting B and C be the respective deflections of the ball and the core in millimeters when compressed under a given load, B≥2.8 and 1.6≤C/B≤2.3, such that the core has an internal hardness which satisfies the condition 1.0≤(Cs−C75)/(C50−Cc)≤2.3, where Cc is the Shore C hardness at the core center, Cs is the Shore C hardness at the core surface, C75 is the Shore C hardness at a position 75% of the core radius from the core center toward the core surface and C50 is the Shore C hardness at a position 50% of the core radius from the core center to the core surface, and such that the core satisfies the condition 630≤Core vh≤1,000, where Core vh is the product of the core volume V (cm³) multiplied by C50. Namely, a good distance can be achieved by the ball on full shots with an iron owing to a spin rate-lowering effect and a very soft and good feel can be obtained at impact, in addition to which the durability to cracking on repeated impact and the controllability of the ball around the green are both good.

In other words, the golf ball of the invention is a spin-type golf ball having three or more cover layers which is dedicated to achieving a good distance on full shots with an iron and which moreover is receptive to spin in the short game and thus satisfies the needs of users who desire controllability around the green. In addition, a soft and good feel can be obtained on all shots with the golf ball of the invention. Skillfully striking a golf ball with a driver (W #1) is generally difficult; the ball may or may not travel well depending on how it is hit. However, the golf ball of this invention is a ball which can reliably achieve a good distance at least on shots with an iron. Hence, it is a golf ball which, by being dedicated to achieving a good distance on iron shots, is targeted at this type of user.

Accordingly, the invention provides a multi-piece solid golf ball having a core, an envelope layer, an intermediate layer and a cover, wherein the core is formed of a rubber composition as one or more layer; the envelope layer is formed of a resin material as one or more layer; the intermediate layer is formed of a resin material as one layer; and the cover is formed of a resin material as one layer having a thickness of not more than 1.0 mm. The Shore C hardness at a surface of the core, the Shore C hardness at a surface of the sphere obtained by encasing the core with the envelope layer (envelope layer-encased sphere), the Shore C hardness at a surface of the sphere obtained by encasing the envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) and the Shore C hardness at a surface of the ball together satisfy the conditions (Shore C hardness at surface of envelope layer-encased sphere)>(Shore C hardness at core surface) and (Shore C hardness at surface of intermediate layer-encased sphere)>(Shore C hardness at ball surface). Letting C be the deflection of the core in millimeters when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and B be the deflection of the ball in millimeters when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), B≥2.8 and 1.6≤C/B≤2.3. The core has an internal hardness which satisfies the condition:

1.0≤(Cs−C75)/(C50−Cc)≤2.3,

where Cc is the Shore C hardness at the core center, Cs is the Shore C hardness at the core surface, C75 is the Shore C hardness at a position 75% of the core radius from the core center toward the core surface and C50 is the Shore C hardness at a position 50% of the core radius from the core center to the core surface. Also, the core satisfies the condition:

630≤Core vh≤1,000,

where Core vh is the product of the core volume V (cm³) multiplied by C50.

In a preferred embodiment of the golf ball of the invention, the interior hardness of the core satisfies the condition:

3.0≤(Cs−C50)/(C25−Cc)≤20.0,

where C25 is the Shore C hardness at a position 25% of the core radius from the core center toward the core surface.

In another preferred embodiment of the invention, the interior hardness of the core satisfies the condition:

(Cs−C50)/(C50−Cc)≥1.1.

In yet another preferred embodiment, the Shore C hardness at the surface of the envelope layer-encased sphere and the Shore C hardness at the surface of the intermediate layer-encased sphere together satisfy the condition:

(Shore C hardness at surface of intermediate layer-encased sphere)>(Shore C hardness at surface of envelope layer-encased sphere).

In still another preferred embodiment, the layers have respective thicknesses which together satisfy the conditions:

(cover thickness)<(intermediate layer thickness)≤(total thickness of envelope layer).

In a further preferred embodiment, the layers have respective thicknesses which together satisfy the condition:

(total thickness of envelope layer)/(cover thickness+intermediate layer thickness)≥1.0.

In a still further preferred embodiment, the envelope layer is formed of at least two layers that include an inner envelope layer and an outer envelope layer. In this embodiment, the Shore C hardnesses at the surfaces of the respective layers may satisfy the following conditions:

(Shore C hardness at ball surface)<(Shore C hardness at surface of intermediate layer-encased sphere)>(Shore C hardness at surface of outer envelope layer-encased sphere)≥(Shore C hardness at surface of inner envelope layer-encased sphere)>(Shore C hardness at core surface).

In yet another preferred embodiment of the golf ball of the invention, the core has a diameter and the ball has a diameter which together satisfy the condition:

0.65≤(core diameter)/(ball diameter)≤0.80.

Advantageous Effects of the Invention

The multi-piece solid golf ball of the invention has a low spin rate on full shots with an iron, enabling it to achieve a good distance, and also has a very soft and good feel. In addition, the durability to cracking on repeated impact and the controllability around the green are both good. Such qualities make this ball particularly useful to golfers who place a premium on the distance traveled on full shots with an iron.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional view of a multi-piece solid golf ball according to the invention which has a five-layer construction.

FIGS. 2A and 2B are, respectively, a top view and a side view of the exterior of a golf ball showing the arrangement of dimples common to all of the Examples and Comparative Examples described herein.

FIG. 3 is a graph showing the core hardness profiles in Examples 1 to 4.

FIG. 4 is a graph showing the core hardness profiles in Comparative Examples 1 to 5 and 7.

FIG. 5 is a graph showing the core hardness profiles in Comparative Examples 6 and 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the appended diagrams.

Referring to FIG. 1 , the multi-piece solid golf ball of the invention is a golf ball G of four or more layers which has a core 1, an envelope layer 2 encasing the core 1, an intermediate layer 3 encasing the envelope layer 2, and a cover 4 encasing the intermediate layer 3. In FIG. 1 , the envelope layer 2 is formed of two layers: an inner envelope layer 2 a and an outer envelope layer 2 b. Numerous dimples D are typically formed on the surface of the cover 4. Although not shown in the diagrams, a coating layer is generally painted onto the surface of the cover 4. Aside from the coating layer, the cover 4 is positioned as the outermost layer in the layered construction of the golf ball. Both the core 1 and the envelope layer 2 are not limited to a single layer, and may each be formed as two or more layers. However, the intermediate layer 3 and the cover 4 are each formed as a single layer.

The core has a diameter that is preferably at least 24.7 mm, more preferably at least 25.7 mm, and even more preferably at least 26.7 mm. The upper limit in the core diameter is preferably 34.7 mm or less, more preferably 33.3 mm or less, and even more preferably 31.7 mm or less.

The (core diameter)/(ball diameter) ratio is preferably at least 0.65, more preferably at least 0.67, and even more preferably at least 0.70. The upper limit is preferably not more than 0.80, more preferably not more than 0.76, and even more preferably not more than 0.73. When this value is too small, the initial velocity of the ball may be low or the deflection hardness of the overall ball may rise, which may result in an increased spin rate on full shots and make it impossible to achieve the intended distance. When this value is too large, the spin rate on full shots with an iron may rise and make it impossible to achieve the intended distance, or the durability to cracking on repeated impact may worsen.

The core has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably at least 5.0 mm, more preferably at least 5.5 mm, and even more preferably at least 6.0 mm. The upper limit is preferably not more than 9.0 mm, more preferably not more than 8.5 mm, and even more preferably not more than 8.0 mm. When the core deflection is too small, i.e., when the core is too hard, the spin rate of the ball may rise excessively so that the ball does not travel far, or the feel at impact may become too hard. On the other hand, when the core deflection is too large, i.e., when the core is too soft, the ball rebound may become too low so that the ball does not travel far, the feel at impact may become too soft, or the durability to cracking on repeated impact may worsen.

The core is obtained by vulcanizing a rubber composition composed primarily of a rubber material. This rubber composition is typically obtained by using a base rubber as the chief component and compounding, together with this, other ingredients such as a co-crosslinking agent, a crosslinking initiator, an inert filler and an organosulfur compound.

It is preferable to use polybutadiene as the base rubber. A commercial product may be used as the polybutadiene. Illustrative examples include BR01, BR51 and BR730 (all from JSR Corporation). The proportion of polybutadiene within the base rubber is preferably at least 60 wt %, and more preferably at least 80 wt %. Rubber ingredients other than the above polybutadiene may be included in the base rubber, provided that doing so does not detract from the advantageous effects of the invention. Examples of rubber ingredients other than the above polybutadiene include other polybutadienes and also other diene rubbers, such as styrene-butadiene rubbers, natural rubbers, isoprene rubbers and ethylene-propylene-diene rubbers.

The co-crosslinking agent is an α,β-unsaturated carboxylic acid and/or a metal salt thereof. Specific examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. The use of acrylic acid or methacrylic acid is especially preferred. Metal salts of unsaturated carboxylic acids include, without particular limitation, the above unsaturated carboxylic acids that have been neutralized with desired metal ions. Specific examples include the zinc salts and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

The unsaturated carboxylic acid and/or metal salt thereof is included in an amount, per 100 parts by weight of the base rubber, which is typically least 5 parts by weight, preferably at least 9 parts by weight, and more preferably at least 13 parts by weight. The amount included is typically not more than 60 parts by weight, preferably not more than 50 parts by weight, and more preferably not more than 40 parts by weight. Too much may make the core too hard, giving the ball an unpleasant feel at impact, whereas too little may lower the rebound.

It is suitable to use an organic peroxide as the crosslinking initiator. Commercially available organic peroxides may be used for this purpose. Examples of such products that may be suitably used include Percumyl D, Perhexa C-40 and Perhexa 3M (all from NOF Corporation), and Luperco 231XL (from AtoChem Co.). One of these may be used alone, or two or more may be used together. The amount of organic peroxide included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, more preferably at least 0.3 part by weight, and even more preferably at least 0.5 part by weight. The upper limit is preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, even more preferably not more than 3 parts by weight, and most preferably not more than 2.5 parts by weight. When too much or too little is included, it may not be possible to obtain a golf ball having a good feel, durability and rebound.

Fillers that may be suitably used include, for example, zinc oxide, barium sulfate and calcium carbonate. One of these may be used alone, or two or more may be used together. The amount of inert filler included per 100 parts by weight of the base rubber is preferably at least 1 part by weight, and more preferably at least 3 parts by weight. The upper limit per 100 parts by weight of the base rubber is preferably not more than 200 parts by weight, more preferably not more than 150 parts by weight, and even more preferably not more than 100 parts by weight. Too much or too little inert filler may make it impossible to obtain a proper weight and a suitable rebound.

Commercial products such as Nocrac NS-6, Nocrac NS-30, Nocrac 200 and Nocrac MB (all available from Ouchi Shinko Chemical Industry Co., Ltd.) may be used as an antioxidant. One of these may be used alone, or two or more may be used together.

The amount of antioxidant included per 100 parts by weight of the base rubber, although not particularly limited, is preferably at least 0.05 part by weight, and more preferably at least 0.1 part by weight. The upper limit is preferably not more than 1.0 part by weight, more preferably not more than 0.7 part by weight, and even more preferably not more than 0.5 part by weight. Too much or too little antioxidant may make it impossible to achieve a suitable core hardness gradient and a suitable rebound, durability and spin rate-lowering effect on full shots.

In addition, an organosulfur compound may be included in the rubber composition so as to impart an excellent rebound. Specifically, it is recommended that thiophenols, thionaphthols, halogenated thiophenols or metal salts of these be included. Specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, the zinc salt of pentachlorothiophenol, and any of the following having 2 to 4 sulfur atoms: diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides. The use of diphenyldisulfide or the zinc salt of pentachlorothiophenol is especially preferred.

The organosulfur compound is included in an amount per 100 parts by weight of the base rubber that is preferably at least 0.05 part by weight, more preferably at least 0.07 part by weight, and even more preferably at least 0.1 part by weight. The upper limit is preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, even more preferably not more than 3 parts by weight, and still more preferably not more than 2 parts by weight. Including too much organosulfur compound may make the hardness too low. On the other hand, including too little may make a rebound-improving effect unlikely.

The core can be produced by vulcanizing and curing the rubber composition containing the above ingredients. For example, the core can be produced by using a Banbury mixer, roll mill or other mixing apparatus to intensively mix the rubber composition, subsequently compression molding or injection molding the mixture in a core mold, and curing the resulting molded body by suitably heating it under conditions sufficient to allow the organic peroxide or co-crosslinking agent to act, such as at a temperature of between 100 and 200° C., preferably between 140 and 180° C., for 10 to 40 minutes.

The core may consist only of one layer or may be formed as two layers consisting of an inner core layer and an outer core layer. When the core is formed as a two-layer core consisting of an inner core layer and an outer core layer, the inner core layer and outer core layer materials may each be composed primarily of the above-described rubber material. Also, the rubber material making up the outer core layer encasing the inner core layer may be the same as or different from the inner core layer material. The details here are the same as those given above for the ingredients of the core-forming rubber material.

Next, the hardness profile of the core is described. The core hardnesses described below refer to Shore C hardnesses. These Shore C hardnesses are hardness values measured with a Shore C durometer in accordance with ASTM D2240.

The core center hardness Cc is preferably 33 or more, more preferably 38 or more, and even more preferably 43 or more. The upper limit is preferably not more than 62, more preferably not more than 59, and even more preferably not more than 54. When this value is too large, the feel at impact may harden or the spin rate on full shots may rise, as a result of which the intended distance may not be attainable. On the other hand, when this value is too small, the rebound may become low and the ball may not travel well, or the durability to cracking on repeated impact may worsen.

The Shore C hardness at a position 25% of the core radius from the core center toward the core surface (C25) is preferably 35 or more, more preferably 40 or more and even more preferably 45 or more. The upper limit is preferably not more than 64, more preferably not more than 61, and even more preferably not more than 56. Outside of these hardness values, undesirable results similar to those described above in connection with the core center hardness (Cc) may arise.

The Shore C hardness at a position 50% of the core radius from the core center toward the core surface (C50) is preferably 37 or more, more preferably 42 or more and even more preferably 47 or more. The upper limit is preferably not more than 66, more preferably not more than 63, and even more preferably not more than 58. Outside of these hardness values, undesirable results similar to those described above in connection with the core center hardness (Cc) may arise. This Shore C hardness C50 corresponds to the hardness at the midpoint between the core surface and the core center (Cm).

The Shore C hardness at a position 75% of the core radius from the core center toward the core surface (C75) is preferably 40 or more, more preferably 45 or more and even more preferably 49 or more. The upper limit is preferably not more than 69, more preferably not more than 65, and even more preferably not more than 61. Outside of these hardness values, undesirable results similar to those described above in connection with the core center hardness (Cc) may arise.

The core surface hardness Cs is preferably 45 or more, more preferably 50 or more, and even more preferably 55 or more. The upper limit is preferably not more than 74, more preferably not more than 70, and even more preferably not more than 66. Outside of these hardness values, undesirable results similar to those described above in connection with the core center hardness (Cc) may arise.

The difference between the core surface hardness (Cs) and the core center hardness (Cc) is preferably 5 or more, more preferably 7 or more, and even more preferably 10 or more. The upper limit is preferably not more than 25, more preferably not more than 20, and even more preferably not more than 15. When this difference is too small, the spin rate on full shots with a driver may rise, as a result of which the intended distance may not be achieved. When this difference is too large, the durability to cracking on repeated impact may worsen, or the initial velocity of the ball when struck may be low, as a result of which the intended distance may not be achieved.

It is critical for the core interior hardness to be such that the value of (Cs−C75)/(C50−Cc) is 1.0 or more, preferably 1.2 or more, and more preferably 1.3 or more; the upper limit is not more than 2.3, preferably not more than 2.0, and more preferably not more than 1.8. When this value is too large, the durability to cracking on repeated impact may worsen or the initial velocity of the ball when struck may be low, as a result of which the intended distance may not be achieved. On the other hand, when this value is too small, the spin rate on full shots may rise, as a result of which the intended distance may not be achieved.

The core interior hardness is such that the value of (Cs−C50)/(C25−Cc) is preferably 3.0 or more, more preferably 3.3 or more, and even more preferably 3.6 or more; the upper limit is preferably not more than 20.0, more preferably not more than 13.0, and even more preferably not more than 7.0. When this value is too large, the durability to cracking on repeated impact may worsen or the initial velocity of the ball when struck may be low, as a result of which the intended distance may not be achieved. On the other hand, when this value is too small, the spin rate on full shots may rise, as a result of which the intended distance may not be achieved.

The core interior hardness is such that the value of (Cs−C50)/(C50−Cc) is preferably 1.1 or more, more preferably 1.3 or more, and even more preferably 1.6 or more; the upper limit is preferably not more than 8.0, and more preferably not more than 4.0. When this value is too large, the durability to cracking on repeated impact may worsen or the initial velocity of the ball when struck may be low, as a result of which the intended distance may not be achieved. On the other hand, when this value is too small, the spin rate on full shots may rise, as a result of which the intended distance may not be achieved.

The core used in this invention has a Core vh value, defined here as the product of the core volume V (cm³) multiplied by C50, which must be 630 or more and is preferably 650 or more, and more preferably 680 or more. The upper limit is not more than 1,000, preferably not more than 950, and more preferably not more than 900. When this value is too large, the spin rate on full shots rises excessively, as a result of which the intended distance cannot be achieved. On the other hand, when this value is too small, the initial velocity of the ball when struck is too low, as a result of which the intended distance cannot be achieved.

Next, the envelope layer is described.

In this invention, the envelope layer may be formed as one layer or as a plurality of two or more layers. When the envelope layer is formed as two or more layers, the layer positioned on the innermost side is called the inner envelope layer and the layer positioned on the outermost side is called the outer envelope layer. In cases where the envelope layer consists of a plurality of layers, the material and surface hardnesses of the envelope layer mentioned below refer to the material hardness and surface hardness of the outer envelope layer.

The envelope layer has a material hardness which is not particularly limited. The material hardness on the Shore C hardness scale is preferably at least 67, more preferably at least 70, and even more preferably at least 72. The upper limit is preferably not more than 90, more preferably not more than 89, and even more preferably not more than 88. The material hardness on the Shore D hardness scale is preferably at least 43, more preferably at least 45, and even more preferably at least 47. The upper limit is preferably not more than 60, more preferably not more than 56, and even more preferably not more than 54.

The sphere obtained by encasing the core with the envelope layer (envelope layer-encased sphere) has a surface hardness which, on the Shore C hardness scale, is preferably at least 75, more preferably at least 78, and even more preferably at least 80. The upper limit is preferably not more than 95, more preferably not more than 93, and even more preferably not more than 92. The surface hardness on the Shore D hardness scale is preferably at least 49, more preferably at least 51, and even more preferably at least 53. The upper limit is preferably not more than 66, more preferably not more than 62, and even more preferably not more than 60.

When the material hardness and surface hardness of the envelope layer are softer than the above ranges, the ball may be too receptive to spin on full shots or the initial velocity may decrease, resulting in a poor distance. On the other hand, when the material hardness and surface hardness of the envelope layer are higher than the above ranges, the feel of the ball at impact may become hard, the durability to cracking on repeated impact may worsen, or the spin rate on full shots may rise, as a result of which a good distance may not be obtained.

In cases where the envelope layer consists of a plurality of layers, it is preferable for the surface hardness of the outer envelope layer to be higher than the surface hardness of the inner envelope layer. In cases where one or more layer is interposed between the inner envelope layer and the outer envelope layer, it is preferable for the ball to be designed in such a way that the layers become progressively harder from the inner envelope layer to the outer envelope layer. When such is not the case, the spin rate on full shots may rise and a good distance may not be achieved. By thus designing the envelope layer so as to consist of a plurality layers, owing to a spin rate-lowering effect on full shots, a better distance is sometimes achieved than when the envelope layer consists of a single layer.

The envelope layer has a thickness which is preferably at least 2.0 mm, more preferably at least 2.7 mm, and even more preferably at least 3.5 mm. The upper limit in the thickness of the envelope layer is preferably not more than 7.0 mm, more preferably not more than 6.5 mm, and even more preferably not more than 6.0 mm. As used herein, the thickness of the envelope layer refers to the total thickness of the constituent layers when the envelope layer consists of a plurality of layers. If the envelope layer is too thin, the spin rate-lowering effect on full shots with an iron may be inadequate and the intended distance may not be obtained. When the envelope layer is too thick, the initial velocity of the overall ball may decline and the initial velocity on shots may become too low, as a result of which the intended distance may not be achieved.

The thickness of the envelope layer preferably satisfies the following condition in the thickness relationship with the subsequently described intermediate layer and cover: (cover thickness)<(intermediate layer thickness)≤(total thickness of envelope layers). Also, the thicknesses of the respective layers have a ratio therebetween, expressed as (total thickness of envelope layers)/(cover thickness+intermediate layer thickness), whose value is preferably at least 1.0, more preferably at least 1.5, and even more preferably at least 1.8; the upper limit is preferably not more than 3.5, more preferably not more than 3.0, and even more preferably not more than 2.6. When this value is too large, the initial velocity may decrease and a good distance may not be obtained, or the durability to cracking on repeated impact may worsen. On the other hand, when this value is too small, the spin rate-lowering effect may be inadequate and a good distance may not be achieved.

The envelope layer material is not particularly limited, although a known resin may be used for this purpose. Examples of especially preferred materials include resin compositions formulated from:

a base resin of (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer blended with (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio between 100:0 and 0:100, and

(c) a non-ionomeric thermoplastic elastomer

in a weight ratio between 100:0 and 50:50.

The intermediate layer-forming resin material described in, for example, JP-A 2010-253268 may be suitably used as above components (a) to (c).

Depending on the intended use, optional additives may be suitably included in the envelope layer-forming resin material. For example, pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added. When these additives are included, the amount added per 100 parts by weight of the overall base resin is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight. The upper limit is preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.

Next, the intermediate layer is described.

The intermediate layer has a material hardness on the Shore D hardness scale which, although not particularly limited, is preferably at least 58, more preferably at least 60, and even more preferably at least 63. The upper limit is preferably not more than 70, more preferably not more than 68, and even more preferably not more than 65. The material hardness on the Shore C hardness scale is preferably at least 87, more preferably at least 89, and even more preferably at least 93. The upper limit is preferably not more than 100, more preferably not more than 98, and even more preferably not more than 96.

The sphere obtained by encasing the envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) has a surface hardness which, on the Shore D hardness scale, is preferably at least 64, more preferably at least 66, and even more preferably at least 69. The upper limit is preferably not more than 76, more preferably not more than 74, and even more preferably not more than 71. The surface hardness on the Shore C hardness scale is preferably at least 90, more preferably at least 93, and even more preferably at least 96. The upper limit is preferably not more than 100, more preferably not more than 99, and even more preferably not more than 98.

When the material and surface hardnesses of the intermediate layer are lower than the above ranges, the ball may be too receptive to spin on full shots or the initial velocity may become low, as a result of which a good distance may not be achieved. On the other hand, when the material and surface hardnesses of the intermediate layer are higher than the above ranges, the durability to cracking on repeated impact may worsen or the feel at impact on shots with a putter or on short approaches may become too hard.

Also, it is desirable for the surface hardness of the intermediate layer-encased sphere to satisfy the following condition with respect to the surface hardness of the ball:

(surface hardness of intermediate layer-encased sphere)>(surface hardness of ball).

When this is not the case, the spin rate of the ball on full shots may rise and a good distance may not be achieved, or the controllability of the ball in the short game may worsen.

The intermediate layer has a thickness which is preferably at least 0.7 mm, more preferably at least 0.8 mm, and even more preferably at least 1.0 mm. The intermediate layer thickness has an upper limit that is preferably not more than 1.8 mm, more preferably not more than 1.4 mm, and even more preferably not more than 1.2 mm. The intermediate layer is preferably thicker than the subsequently described cover (outermost layer). When the intermediate layer has a thickness which falls outside of the above range or is formed thinner than the cover, the spin rate-lowering effect on shots with a driver (W #1) may be inadequate, which may result in a poor distance. Also, when the intermediate layer is too thin, the durability to cracking on repeated impact and the low-temperature durability may worsen.

The intermediate layer material may be suitably selected from among various thermoplastic resins that are used as golf ball materials, with the use of the highly neutralized resin material containing components (a) to (c) described above in connection with the envelope layer material or an ionomer resin being preferred.

Specific examples of ionomer resin materials include sodium-neutralized ionomer resins and zinc-neutralized ionomer resins. These may be used singly or two or more may be used together.

An embodiment that uses in admixture a zinc-neutralized ionomer resin and a sodium-neutralized ionomer resin as the chief material is especially preferred. The blending ratio therebetween, expressed as the weight ratio (zinc-neutralized ionomer)/(sodium-neutralized ionomer), is from 25/75 to 75/25, preferably from 35/65 to 65/35, and more preferably from 45/55 to 55/45. When the zinc-neutralized ionomer and sodium-neutralized ionomer are not included in a ratio within this range, the rebound may become too low and the desired distance may not be achieved, the durability to cracking on repeated impact at normal temperatures may worsen, or the durability to cracking at low temperatures (subzero Centigrade) may worsen.

The resin material used to form the intermediate layer may be one obtained by blending, of commercially available ionomer resins, a high-acid ionomer resin having an acid content of at least 16 wt % with an ordinary ionomer resin. The high rebound and lower spin rate resulting from the use of such a blend enables a good distance to be achieved on shots with a driver (W #1).

The amount of unsaturated carboxylic acid included in the high-acid ionomer resin (acid content) is typically at least 16 wt %, preferably at least 17 wt %, and more preferably at least 18 wt %. The upper limit is preferably not more than 22 wt %, more preferably not more than 21 wt %, and even more preferably not more than 20 wt %. When this value is too small, the spin rate on full shots may rise, as a result of which the intended distance may not be attainable. On the other hand, when this value is too large, the feel on impact may become too hard, or the durability to cracking on repeated impact may worsen.

The amount of high-acid ionomer resin included per 100 wt % of the resin material is preferably at least 10 wt %, more preferably at least 30 wt %, and even more preferably at least 60 wt %. When the content of this high-acid ionomer resin is too low, the spin rate on shots with a driver (W #1) may rise and a good distance may not be attained.

Depending on the intended use, optional additives may be suitably included in the intermediate layer material. For example, pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added. When these additives are included, the amount added per 100 parts by weight of the base resin is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight. The upper limit is preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.

It is desirable to abrade the surface of the intermediate layer in order to increase adhesion of the intermediate layer material with the polyurethane that is preferably used in the subsequently described cover material. In addition, it is desirable to apply a primer (adhesive) to the surface of the intermediate layer following such abrasion treatment or to add an adhesion reinforcing agent to the intermediate layer material.

The intermediate layer material has a specific gravity which is typically less than 1.1, preferably between 0.90 and 1.05, and more preferably between 0.93 and 0.99. Outside of this range, the rebound of the overall ball may decrease and a good distance may not be obtained, or the durability of the ball to cracking on repeated impact may worsen.

Next, the cover (outermost layer) is described.

The cover has a material hardness on the Shore D hardness scale which, although not particularly limited, is preferably at least 35, more preferably at least 40, and even more preferably at least 45. The upper limit is preferably not more than 60, more preferably not more than 55, and even more preferably not more than 50. The material hardness on the Shore C hardness scale is preferably at least 57, more preferably at least 63, and even more preferably at least 70. The upper limit is preferably not more than 89, more preferably not more than 83, and even more preferably not more than 76.

The sphere obtained by encasing the intermediate layer-encased sphere with the cover (i.e. the ball) has a surface hardness on the Shore D hardness scale that is preferably at least 50, more preferably at least 53, and even more preferably at least 56. The upper limit is preferably not more than 70, more preferably not more than 67, and even more preferably not more than 64. The material hardness on the Shore C hardness scale is preferably at least 75, more preferably at least 80, and even more preferably at least 85. The upper limit is preferably not more than 95, more preferably not more than 92, and even more preferably not more than 90.

When the material hardness of the cover and the ball surface hardness are lower than the above respective ranges, the spin rate of the ball on full shots with an iron may rise and a good distance may not be achieved. On the other hand, when the material hardness of the cover and the ball surface hardness are too high, the ball may not be receptive to spin on approach shots or the scuff resistance may worsen.

The cover has a thickness of preferably at least 0.3 mm, more preferably at least 0.45 mm, and even more preferably at least 0.6 mm. The upper limit in the cover thickness is preferably not more than 1.0 mm, more preferably not more than 0.9 mm, and even more preferably not more than 0.85 mm. When the cover is too thick, the rebound on full shots with an iron may become inadequate or the spin rate may rise, as a result of which a good distance may not be achieved. On the other hand, when the cover is too thin, the scuff resistance may worsen or the ball may not be receptive to spin on approach shots and may thus lack sufficient controllability.

Various thermoplastic resins and thermoset resins employed as cover stock in golf balls may be used as the cover material. For reasons having to do with controllability and scuff resistance, preferred use can be made of a urethane resin. In particular, from the standpoint of the mass productivity of the manufactured balls, it is preferable to use a material that is composed primarily of a thermoplastic polyurethane, and more preferable to form the cover of a resin blend in which the main components are (I) a thermoplastic polyurethane and (II) a polyisocyanate compound.

It is recommended that the total weight of components (I) and (II) combined be at least 60%, and preferably at least 70%, of the overall amount of the cover-forming resin composition. Components (I) and (II) are described in detail below.

The thermoplastic polyurethane (I) has a structure which includes soft segments composed of a polymeric polyol (polymeric glycol) that is a long-chain polyol, and hard segments composed of a chain extender and a polyisocyanate compound. Here, the long-chain polyol serving as a starting material may be any that has hitherto been used in the art relating to thermoplastic polyurethanes, and is not particularly limited. Illustrative examples include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly, or two or more may be used in combination. Of these, in terms of being able to synthesize a thermoplastic polyurethane having a high rebound resilience and excellent low-temperature properties, a polyether polyol is preferred.

Any chain extender that has hitherto been employed in the art relating to thermoplastic polyurethanes may be suitably used as the chain extender. For example, low-molecular-weight compounds with a molecular weight of 400 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups are preferred. Illustrative, non-limiting, examples of the chain extender include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Of these, the chain extender is preferably an aliphatic diol having from 2 to 12 carbon atoms, and is more preferably 1,4-butylene glycol.

Any polyisocyanate compound hitherto employed in the art relating to thermoplastic polyurethanes may be suitably used without particular limitation as the polyisocyanate compound. For example, use may be made of one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. However, depending on the type of isocyanate, the crosslinking reactions during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use the following aromatic diisocyanate: 4,4′-diphenylmethane diisocyanate.

Commercially available products may be used as the thermoplastic polyurethane serving as component (I). Illustrative examples include Pandex T-8295, Pandex T-8290 and Pandex T-8260 (all from DIC Covestro Polymer, Ltd.).

A thermoplastic elastomer other than the above thermoplastic polyurethanes may also be optionally included as a separate component, i.e., component (III), together with above components (I) and (II). By including this component (III) in the above resin blend, the flowability of the resin blend can be further improved and properties required of the golf ball cover material, such as resilience and scuff resistance, can be increased.

The compositional ratio of above components (I), (II) and (III) is not particularly limited. However, to fully elicit the advantageous effects of the invention, the compositional ratio (I):(II):(III) is in a weight ratio range of preferably between 100:2:50 and 100:50:0, and more preferably between 100:2:50 and 100:30:8.

In addition, various additives other than the ingredients making up the above thermoplastic polyurethane may be optionally included in this resin blend. For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and internal mold lubricants may be suitably included.

The manufacture of multi-piece solid golf balls in which the above-described core, envelope layer, intermediate layer and cover (outermost layer) are formed as successive layers may be carried out by a customary method such as a known injection molding process. For example, a multi-piece golf ball can be produced by successively injection-molding the respective materials for the envelope layer and the intermediate layer over the core in injection molds for each layer so as to obtain the respective layer-encased spheres and then, last of all, injection-molding the material for the cover serving as the outermost layer over the intermediate layer-encased sphere. Alternatively, the encasing layers may each be formed by enclosing the sphere to be encased within two half-cups that have been pre-molded into hemispherical shapes and then molding under applied heat and pressure.

The golf ball has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which must be at least 2.8 mm, and is preferably at least 3.1 mm, and more preferably at least 3.4 mm. The deflection upper limit value is preferably not more than 4.5 mm, more preferably not more than 4.2 mm, and even more preferably not more than 4.0 mm. When the deflection by the golf ball is too small, i.e., when the ball is too hard, the spin rate rises excessively so that the ball does not achieve a good distance, or the feel at impact is too hard. On the other hand, when the deflection is too large, i.e., when the ball is too soft, the ball rebound may be too low so that the ball does not achieve a good distance, the feel at impact may be too soft, or the durability to cracking under repeated impact may worsen.

Letting C be the deflection of the core in millimeters when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and B be the deflection of the ball in millimeters when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), the value of CIB must be 1.6 or more, and is preferably 1.7 or more. The upper limit value must be no more than 2.3, and is preferably not more than 2.1. When this value is too large, the initial velocity on shots becomes low and a good distance cannot be achieved, or the durability to cracking on repeated impact worsens. When this value is too small, the ball spin rate-lowering effect is inadequate and a good distance cannot be achieved.

Also, the value of C−B is preferably at least 2.5 mm, more preferably at least 2.6 mm, and even more preferably at least 2.7 mm; the upper limit is preferably not more than 5.0 mm, more preferably not more than 4.6 mm, and even more preferably not more than 4.3 mm. When this value is too large, the initial velocity on shots may become low and a good distance may not be achieved, or the durability to cracking on repeated impact may worsen. When this value is too small, the spin rate-lowering effect may be inadequate and a good distance may not be achieved.

Hardness Relationships Among Layers

In the invention, it is critical that the Shore C hardness at the core surface, the Shore C hardness at the surface of the sphere obtained by encasing the core with the envelope layer (envelope layer-encased sphere), the Shore C hardness at the surface of the sphere obtained by encasing the envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) and the Shore C hardness at the ball surface together satisfy the conditions (Shore C hardness at surface of envelope layer-encased sphere)>(Shore C hardness at core surface) and (Shore C hardness at surface of intermediate layer-encased sphere)>(Shore C hardness at ball surface).

The value obtained by subtracting the surface hardness of the core from the surface hardness of the envelope layer-encased sphere, expressed on the Shore C hardness scale, is larger than 0, preferably 8 or more, and more preferably 15 or more. The upper limit value is preferably not more than 50, more preferably not more than 43, and even more preferably not more than 36. When this value falls outside of the above numerical range, the spin rate of the ball on full shots may rise and the intended distance may not be attainable.

The value obtained by subtracting the surface hardness of the ball from the surface hardness of the intermediate layer-encased sphere, expressed on the Shore C hardness scale, is larger than 0, preferably 3 or more, and more preferably 5 or more. The upper limit value is preferably not more than 30, more preferably not more than 22, and even more preferably not more than 15. When this value is too small, the controllability in the short game may worsen. On the other hand, when this value is too large, the spin rate on full shots may rise, as a result of which the intended distance may not be attainable.

The value obtained by subtracting the core center hardness from the surface hardness of the envelope layer-encased sphere, expressed on the Shore C hardness scale, is preferably 29 or more, more preferably 32 or more, and even more preferably 35 or more. The upper limit value is preferably not more than 55, more preferably not more than 50, and even more preferably not more than 45. When this value is too large, the durability to cracking on repeated impact may worsen or the initial velocity on shots may become low, as a result of which the intended distance may not be attainable. On the other hand, when this value is too small, the spin rate on full shots may rise, as a result of which the intended distance may not be attainable.

The value obtained by subtracting the surface hardness of the envelope layer-encased sphere from the surface hardness of the intermediate layer-encased sphere, expressed on the Shore C hardness scale, is preferably greater than 0, more preferably 4 or more, and even more preferably 8 or more. The upper limit value is preferably not more than 28, more preferably not more than 22, and even more preferably not more than 16. When this value falls outside of the above numerical range, the spin rate on full shots may rise, as a result of which the intended distance may not be attainable.

The value obtained by subtracting the core center hardness from the surface hardness of the intermediate layer-encased sphere, expressed on the Shore C hardness scale, is preferably 33 or more, more preferably 38 or more, and even more preferably 43 or more. The upper limit value is preferably not more than 65, more preferably not more than 60, and even more preferably not more than 55. When this value is too large, the durability to cracking on repeated impact may worsen or the initial velocity on shots may become low, as a result of which the intended distance may not be attainable. On the other hand, when this value is too small, the spin rate on full shots may rise, as a result of which the intended distance may not be attainable.

Relationship Between Volumes and Hardnesses of Core and of Envelope Layer

Letting Core vh be the product of the core volume (cm³) multiplied by the Shore C hardness Cm at the midpoint between the core surface and the core center, IE vh be the product of the volume of the inner envelope layer (cm³) multiplied by the Shore C hardness at the surface of the inner envelope layer-encased sphere and OE vh be the product of the volume of the outer envelope layer (cm³) multiplied by the Shore C hardness at the surface of the outer envelope layer-encased sphere, the value of (OE vh+IE vh)/Core vh is preferably 1.0 or more, more preferably 1.2 or more, and even more preferably 1.4 or more. The upper limit value is preferably not more than 2.3, more preferably not more than 2.1, and even more preferably not more than 1.9. When this value is too large, the initial velocity on shots may decrease and a good distance may not be achieved, or the durability to cracking on repeated impact may worsen. On the other hand, when this value is too small, the spin rate-lowering effect may be inadequate and so a good distance may not be achieved.

Numerous dimples may be formed on the outside surface of the cover (i.e., the outermost layer of the ball). The number of dimples arranged on the cover surface, although not particularly limited, is preferably at least 250, more preferably at least 300, and even more preferably at least 320. The upper limit is preferably not more than 380, more preferably not more than 350, and even more preferably not more than 340. When the number of dimples is higher than this range, the ball trajectory may become lower and the distance traveled by the ball may decrease. On the other hand, when the number of dimples is lower that this range, the ball trajectory may become higher and a good distance may not be achieved.

The dimple shapes used may be of one type or may be a combination of two or more types suitably selected from among, for example, circular shapes, various polygonal shapes, dewdrop shapes and oval shapes. When circular dimples are used, the dimple diameter may be set to at least about 2.5 mm and up to about 6.5 mm, and the dimple depth may be set to at least 0.08 mm and up to 0.30 mm.

In order for the aerodynamic properties to be fully manifested, it is desirable for the dimple coverage ratio on the spherical surface of the golf ball, i.e., the dimple surface coverage SR, which is the sum of the individual dimple surface areas, each defined by the flat plane circumscribed by the edge of a dimple, as a percentage of the spherical surface area of the ball were the ball to have no dimples thereon, to be set to at least 70% and not more than 90%. Also, to optimize the ball trajectory, it is desirable for the value Vo, defined as the spatial volume of the individual dimples below the flat plane circumscribed by the dimple edge, divided by the volume of the cylinder whose base is the flat plane and whose height is the maximum depth of the dimple from the base, to be set to at least 0.35 and not more than 0.80. Moreover, it is preferable for the ratio VR of the sum of the volumes of the individual dimples, each formed below the flat plane circumscribed by the edge of a dimple, with respect to the volume of the ball sphere were the ball surface to have no dimples thereon, to be set to at least 0.6% and not more than 1.0%. Outside of the above ranges in these respective values, the resulting trajectory may not enable a good distance to be achieved and so the ball may fail to travel a fully satisfactory distance.

A coating layer may be formed on the surface of the cover. This coating layer can be formed by applying various types of coating materials. Because the coating layer must be capable of enduring the harsh conditions of golf ball use, it is desirable to use a coating composition in which the chief component is a urethane coating material composed of a polyol and a polyisocyanate.

The polyol component is exemplified by acrylic polyols and polyester polyols. These polyols include modified polyols. To further increase workability, other polyols may also be added.

It is suitable to use two types of polyester polyols together as the polyol component. In this case, letting the two types of polyester polyol be component (a) and component (b), a polyester polyol in which a cyclic structure has been introduced onto the resin skeleton may be used as the polyester polyol of component (a). Examples include polyester polyols obtained by the polycondensation of a polyol having an alicyclic structure, such as cyclohexane dimethanol, with a polybasic acid; and polyester polyols obtained by the polycondensation of a polyol having an alicyclic structure with a diol or triol and a polybasic acid. A polyester polyol having a branched structure may be used as the polyester polyol of component (b). Examples include polyester polyols having a branched structure, such as NIPPOLAN 800, from Tosoh Corporation.

The polyisocyanate is exemplified without particular limitation by commonly used aromatic, aliphatic, alicyclic and other polyisocyanates. Specific examples include tolylene diisocyanate, diphenylmethane diisocyanate, xylylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, lysine diisocyanate, isophorone diisocyanate, 1,4-cyclohexylene diisocyanate, naphthalene diisocyanate, trimethylhexamethylene diisocyanate, dicyclohexylmethane diisocyanate and 1-isocyanato-3,3,5-trimethyl-4-isocyanatomethylcyclohexane. These may be used singly or in admixture.

Depending on the coating conditions, various types of organic solvents may be mixed into the coating composition. Examples of such organic solvents include aromatic solvents such as toluene, xylene and ethylbenzene; ester solvents such as ethyl acetate, butyl acetate, propylene glycol methyl ether acetate and propylene glycol methyl ether propionate; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; ether solvents such as diethylene glycol dimethyl ether, diethylene glycol diethyl ether and dipropylene glycol dimethyl ether; alicyclic hydrocarbon solvents such as cyclohexane, methyl cyclohexane and ethyl cyclohexane; and petroleum hydrocarbon solvents such as mineral spirits.

The thickness of the coating layer made of the coating composition, although not particularly limited, is typically from 5 to 40 μm, and preferably from 10 to 20 μm. As used herein, “coating layer thickness” refers to the coating thickness obtained by averaging the measurements taken at a total of three places: the center of a dimple and two places located at positions between the dimple center and the dimple edge.

In this invention, the coating layer composed of the above coating composition has an elastic work recovery that is preferably at least 60%, and more preferably at least 80%. At a coating layer elastic work recovery in this range, the coating layer has a high elasticity and so the self-repairing ability is high, resulting in an outstanding abrasion resistance. Moreover, the performance attributes of golf balls coated with this coating composition can be improved. The method of measuring the elastic work recovery is described below.

The elastic work recovery is one parameter of the nanoindentation method for evaluating the physical properties of coating layers, this being a nanohardness test method that controls the indentation load on a micro-newton (μN) order and tracks the indenter depth during indentation to a nanometer (nm) precision. In prior methods, only the size of the deformation (plastic deformation) mark corresponding to the maximum load could be measured. However, in the nanoindentation method, the relationship between the indentation load and the indentation depth can be obtained by continuous automated measurement. Hence, unlike in the past, there are no individual differences between observers when visually measuring a deformation mark under an optical microscope, and so it is thought that the physical properties of the coating layer can be precisely evaluated. Given that the coating layer on the ball surface is strongly affected by the impact of drivers and other clubs and has a not inconsiderable influence on various golf ball properties, measuring the coating layer by the nanohardness test method and carrying out such measurement to a higher precision than in the past is a very effective method of evaluation.

The hardness of the coating layer, as expressed on the Shore M hardness scale, is preferably 40 or more, and more preferably 60 or more. The upper limit is preferably not more than 95, and more preferably not more than 85. This Shore M hardness is obtained in accordance with ASTM D2240. The hardness of the coating layer, as expressed on the Shore C hardness scale, is preferably 40 or more, and more preferably 50 or more; the upper limit is preferably not more than 80, and more preferably not more than 70. This Shore C hardness is obtained in accordance with ASTM D2240. At coating layer hardnesses that are higher than these ranges, the coating may become brittle when the ball is repeatedly struck, which may make it incapable of protecting the cover layer. On the other hand, coating layer hardnesses that are lower than the above range are undesirable because the ball surface is more easily damaged when striking a hard object.

When the above coating composition is used, the formation of a coating layer on the surface of golf balls manufactured by a commonly known method may be carried out via the steps of preparing the coating composition at the time of application, applying the composition onto the golf ball surface by a conventional coating operation, and drying the applied composition. The coating method is not particularly limited. For example, spray painting, electrostatic painting or dipping may be suitably used.

The multi-piece solid golf ball of the invention can be made to conform to the Rules of Golf for play. The inventive ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm, and to a weight which is preferably between 45.0 and 45.93 g.

The golf ball has an initial velocity, based on The Royal and Ancient Golf Club of St. Andrews (R&A) Rules of Golf, which is generally at least 76.8 m/s, preferably at least 77.0 m/s, and more preferably at least 77.1 m/s, and which has an upper limit of not more than 77.724 m/s. When this initial velocity exceeds 77.724 m/s, it falls outside of the official rules. On the other hand, when the initial velocity is too low, a good distance may not be achieved on full shots.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 4. Comparative Examples 1 to 8

Solid cores were produced by preparing rubber compositions for the Examples and Comparative Examples shown in Table 1, and then molding and vulcanizing the compositions under the vulcanization conditions for each Example shown in Table 1.

In Examples 1 and 2 and Comparative Examples 7 and 8, which are prospective examples, the core is produced in the same way as above based on the formulations shown in Table 1.

TABLE 1 Example Comparative Example 1 2 3 4 1 2 3 4 5 6 7 8 Core Polybutadiene 100 100 100 100 100 100 100 100 100 100 100 100 formulation Zinc acrylate 13.0 18.0 13.0 18.0 8.0 7.0 17.0 32.5 33.2 32.0 18.0 18.0 (pbw) Organic peroxide 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 Zinc stearate 5 5 5 5 5 5 5 0 0 0 5 5 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Zinc oxide 61.8 59.0 63.3 62.5 60.0 98.7 97.4 29.2 28.9 29.3 62.5 45.5 Zinc salt of 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 pentachlorothiophenol Vulcanization Temperature (° C.) 155 155 155 155 155 155 155 155 155 155 155 155 Time (minutes) 16 16 15 15 20 20 15 15 15 15 15 15

Details on the ingredients mentioned in Table 1 are given below.

-   Polybutadiene: Available under the trade name “BR 730” from JSR     Corporation -   Zinc acrylate: “ZN-DA85S” from Nippon Shokubai Co., Ltd. -   Organic Peroxide: A mixture of 1,1-di(t-butylperoxy)cyclohexane and     silica, available under the trade name “Perhexa C-40” from NOF     Corporation -   Zinc stearate: Available under the trade name “Zinc Stearate G” from     NOF Corporation -   Antioxidant: 2,2′-Methylenebis(4-methyl-6-butylphenol), available     under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical     Industry Co., Ltd. -   Zinc oxide: Available as Grade 3 Zinc Oxide from Sakai Chemical Co.,     Ltd. -   Zinc salt of pentachlorothiophenol:     -   Available from Wako Pure Chemical Industries, Ltd.

Formation of Inner and Outer Envelope Layers

Next, in the Examples and the Comparative Examples other than Comparative Examples 4 to 6, an inner envelope layer was formed by injection-molding the inner envelope layer material of formulation No. 1 shown in Table 2 over the core. An outer envelope layer was then formed by injection-molding the outer envelope layer material of formulation No. 1, No. 2 or No. 3 shown in the same table over the inner envelope layer. In Comparative Examples 4 to 6, a single envelope layer (details for which are shown in the “Outer envelope layer” section of Table 5) was formed by injection molding the material of formulation No. 1 or No. 3 in Table 2 over the core.

In Examples 1 and 2 and Comparative Examples 7 and 8, which are prospective examples, the envelope layers are produced in the same way as above based on the formulations shown in Table 2.

Formation of Intermediate Layer and Cover (Outermost Layer)

Next, in all of the Examples and Comparative Examples, an intermediate layer was formed by injection molding the intermediate layer material of formulation No. 4 or No. 5 shown in Table 2 over the envelope layer-encased sphere obtained as described above. A cover (outermost layer) was then formed by injection-molding the cover material of formulation No. 6 or No. 7 shown in Table 2 over the intermediate layer-encased sphere in each Example. A plurality of given dimples common to all of the Examples and Comparative Examples were formed at this time on the surface of the cover.

In Examples 1 and 2 and Comparative Examples 7 and 8, which are prospective examples, the intermediate layer and the cover are produced in the same way as above based on the formulations shown in Table 2.

TABLE 2 Resin material (pbw) No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 HPF 1000 100 64 HPF 2000 100 Himilan 1605 50 36 Himilan 1557 15 Himilan 1706 35 Surlyn 8120 75 Dynaron 6100P 25 Behenic acid 20 Calcium hydroxide 2.3 Calcium stearate 0.15 Zinc stearate 0.15 Trimethylolpropane 1.1 1.1 TPU (1) 100 TPU (2) 100

Trade names of the chief materials mentioned in the table are given below.

-   HPF 1000, HPF 2000: HPF™ from The Dow Chemical Company -   Himilan 1605, Himilan 1557, Himilan 1706:     -   Ionomers available from Dow-Mitsui Polychemicals Co., Ltd. -   Surlyn 8120: An ionomer available from The Dow Chemical Company -   Dynaron 6100P: A hydrogenated polymer available from JSR Corporation -   Behenic acid: NAA222-S (beads) from NOF Corporation -   Calcium hydroxide: Available as “CLS-B” from Shiraishi Calcium     Kaisha, Ltd. -   Trimethylolpropane: Available from Tokyo Chemical Industry Co., Ltd. -   TPU (1), TPU (2): Ether-type thermoplastic polyurethanes available     under the trade name “Pandex” from DIC Covestro Polymer, Ltd.

Eight types of circular dimples are used. The dimples and the dimple pattern are common to all of the Examples and Comparative Examples. Details on the dimples are shown in Table 3 below, and the dimple pattern is shown in FIG. 2 . FIG. 2A is a top view of the dimples, and FIG. 2B is a side view of the same.

TABLE 3 Cylinder Dimple Num- Diameter Depth Volume volume SR VR A ber (mm) (mm) (mm³) ratio (%) (%) A-1 12 4.6 0.118 1.111 0.566 82.3 0.77 A-2 198 4.45 0.117 1.031 0.566 A-3 36 3.85 0.114 0.752 0.566 A-4 12 2.75 0.085 0.286 0.566 A-5 36 4.45 0.126 1.110 0.566 A-6 24 3.85 0.123 0.811 0.566 A-7 6 3.4 0.115 0.558 0.534 A-8 6 3.3 0.115 0.526 0.534 Total 330

Dimple Definitions

-   Edge: Highest place in cross-section passing through center of     dimple. -   Diameter: Diameter of flat plane circumscribed by edge of dimple. -   Depth: Maximum depth of dimple from flat plane circumscribed by edge     of dimple. -   SR: Sum of individual dimple surface areas, each defined by flat     plane circumscribed by edge of dimple, as a percentage of spherical     surface area of ball were it to have no dimples thereon. -   Dimple volume: Dimple volume below flat plane circumscribed by edge     of dimple. -   Cylinder volume ratio: Ratio of dimple volume to volume of cylinder     having same diameter and depth as dimple. -   VR: Sum of volumes of individual dimples formed below flat plane     circumscribed by edge of dimple, as a percentage of volume of ball     sphere were it to have no dimples thereon.

Formation of Coating Layer

Next, using the coating composition shown in Table 4 below, a coating composition common to all the Examples and Comparative Examples was applied with an air spray gun onto the surface of the cover (outermost layer) on which numerous dimples were formed, thereby producing golf balls having a 15 μm-thick coating layer formed thereon.

The above coating is similarly applied in Examples 1 and 2 and Comparative Examples 7 and 8, which are prospective examples, thereby producing golf balls having a 15 μm-thick coating layer formed thereon.

TABLE 4 Coating Base resin Polyester polyol (A) 23 Composition Polyester polyol (B) 15 C (pbw) Organic solvent 62 Curing agent Isocyanate (HMDI isocyanurate) 42 Solvent 58 Molar blending ratio (NCO/OH) 0.89 Coating Elastic work recovery (%) 84 properties Shore M hardness 84 Shore C hardness 63 Thickness (μm) 15

Polyester Polyol (A) Synthesis Example

A reactor equipped with a reflux condenser, a dropping funnel, a gas inlet and a thermometer is charged with 140 parts by weight of trimethylolpropane, 95 parts by weight of ethylene glycol, 157 parts by weight of adipic acid and 58 parts by weight of 1,4-cyclohexanedimethanol, following which the temperature is raised to between 200° C. and 240° C. under stirring and the reaction is effected by 5 hours of heating. This yielded Polyester Polyol (A) having an acid value of 4, a hydroxyl value of 170 and a weight-average molecular weight (Mw) of 28,000.

The Polyester Polyol (A) thus synthesized is then dissolved in butyl acetate, thereby preparing a varnish having a nonvolatiles content of 70 wt %.

The base resin for Coating Composition C in Table 4 is prepared by mixing together 23 parts by weight of the above polyester polyol solution, 15 parts by weight of Polyester Polyol (B) (the saturated aliphatic polyester polyol NIPPOLAN 800 from Tosoh Corporation; weight-average molecular weight (Mw), 1,000; 100% solids) and the organic solvent. This mixture had a nonvolatiles content of 38.0 wt %.

Elastic Work Recovery

The elastic work recovery of the coating material is measured using a coating film sheet having a thickness of 50 μm. The ENT-2100 nanohardness tester from Erionix Inc. is used as the measurement apparatus, and the measurement conditions are as follows.

Indenter: Berkovich indenter (material: diamond; angle α: 65.03°)

Load F: 0.2 mN

Loading time: 10 seconds

Holding time: 1 second

Unloading time: 10 seconds

The elastic work recovery is calculated as follows, based on the indentation work W_(elast) (Nm) due to spring-back deformation of the coating and on the mechanical indentation work W_(total) (Nm).

Elastic work recovery=W _(elast) /W _(total)×100(%)

Shore C Hardness and Shore M Hardness

The Shore C hardness and Shore M hardness in Table 4 above are determined by forming the material being tested into 2 mm thick sheets and stacking three such sheets together to give a test specimen. Measurements are taken using a Shore C durometer and a Shore M durometer in accordance with ASTM D2240.

Various properties of the resulting golf balls, including the internal hardness of the core, the diameters of the core and each layer-encased sphere, the thickness and material hardness of each layer, and the surface hardness of each layer-encased sphere, are evaluated by the following methods. The results are presented in Tables 5 and 6.

Diameters of Core. Inner and Outer Envelope Layer-Encased Spheres and Intermediate Layer-Encased Sphere

The diameters at five random places on the surface are measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single core, inner envelope layer-encased sphere, outer envelope layer-encased sphere or intermediate layer-encased sphere, the average diameter for ten such spheres is determined.

Ball Diameter

The diameter at 15 random dimple-free areas is measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single ball, the average diameter for ten balls is determined.

Deflections of Core, Various Layer-Encased Spheres and Ball

The sphere to be measured, be it a core, any of the various layer-encased spheres or a ball, is placed on a hard plate and the amount of deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) is measured. The amount of deflection refers in each case to the measured value obtained after holding the sphere to be measured isothermally at 23.9° C. The rate at which pressure is applied by the head compressing the core, layer-encased sphere or ball is set to 10 mm/s.

Core Hardness Profile

The indenter of a durometer is set substantially perpendicular to the spherical surface of the core, and the core surface hardness on the Shore C hardness scale is measured in accordance with ASTM D2240. The P2 Automatic Rubber Hardness Tester (Kobunshi Keiki Co., Ltd.) equipped with a Shore C durometer can be used for measuring the hardness. The maximum value is read off as the hardness value. Measurements are all carried out in a 23±2° C. environment. Cross-sectional hardnesses at specific positions in each core, these hardnesses being the core center hardness Cc, the core surface hardness Cs, the hardness C75 at the position 75% of the core radius from the core center toward the core surface, the hardness C50 at the position 50% of the core radius from the core center toward the core surface and the hardness C25 at the position 25% of the core radius from the core center toward the core surface, are measured by perpendicularly pressing the indenter of a durometer at the place to be measured on the flat cross-section obtained by cutting the core into hemispheres. The results are indicated as Shore C hardness values.

The core hardness profiles of each Example and Comparative Example are shown in the graphs in FIG. 3 (Examples 1 to 5), FIG. 4 (Comparative Examples 1 to 5 and 7) and FIG. 5 (Comparative Examples 6 and 8).

Material Hardnesses (Shore C and D Hardnesses) of Inner and Outer Envelope Layers, Intermediate Layer and Cover

The resin material for each layer is molded into a sheet having a thickness of 2 mm and left to stand for at least two weeks at 23±2° C. Three such sheets are stacked together at the time of measurement. The Shore C hardness and Shore D hardness of each material are measured with, respectively, a Shore C durometer and a Shore D durometer in accordance with ASTM D2240. The P2 Automatic Rubber Hardness Tester (Kobunshi Keiki Co., Ltd.) on which a Shore C durometer or a Shore D durometer has been mounted can be used for measuring the hardness. The maximum value is read off as the hardness value.

Surface Hardnesses (Shore C and Shore D) of Inner and Outer Envelope Layer-Encased Spheres, Intermediate Layer-Encased Sphere and Ball

These hardnesses are measured by perpendicularly pressing an indenter against the surfaces of the respective spheres. The surface hardness of a ball (cover) is the value measured at a dimple-free area (land) on the surface of the ball. The Shore C hardness and Shore D hardness in each case are measured with, respectively, a Shore C durometer and a Shore D durometer in accordance with ASTM D2240. The P2 Automatic Rubber Hardness Tester (Kobunshi Keiki Co., Ltd.) on which a Shore C durometer or a Shore D durometer has been mounted can be used for measuring the hardness. The maximum value is read off as the hardness value.

Ball Initial Velocity

The initial velocity of the ball is measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The ball is tested in a chamber at a room temperature of 23+2° C. after being held isothermally at a temperature of 23±1° C. for at least 3 hours. One dozen balls are each hit four times using a 250-pound (113.4 kg) head (striking mass) at an impact velocity of 143.8 ft/s (43.83 m/s), and the time taken for the balls to traverse a distance of 6.28 ft (1.91 m) is measured and used to compute the initial velocity (m/s). This cycle is carried out over a period of about 15 minutes.

TABLE 5 Example Comparative Example 1 2 3 4 1 2 Core Diameter (mm) 30.71 30.83 30.23 30.36 30.30 26.73 Weight (g) 20.8 20.9 20.0 20.3 20.2 15.7 Volume V (cm³) 15.2 15.3 14.5 14.6 14.6 10.0 Deflection (mm) 7.5 6.3 7.5 6.3 8.8 8.0 Shore C hardness at surface: Cs 56.4 65.3 56.4 65.3 46.1 50.9 Hardness at position 75% of 49.3 60.2 49.3 60.2 40.8 45.4 core radius from center: C75 Hardness at position 50% of 47.8 58.6 47.7 57.3 39.2 43.7 core radius from center: C50 Hardness at position 25% of 45.1 55.1 45.1 55.1 38.9 43.6 core radius from center: C25 Shore C hardness at center: Cc 43.8 53.3 43.8 53.3 38.6 42.0 Shore C hardness difference 12.6 12.0 12.6 12.0 7.5 8.9 between surface and center (Cs − Cc) (Cs − C75)/(C50 − Cc) 1.8 1.0 1.3 1.3 8.8 3.2 (Cs − C50)/(C25 − Cc) 6.6 3.7 6.7 4.4 23.0 4.5 (Cs − C50)/(C50 − Cc) 2.1 1.3 2.2 2.0 12.6 4.4 Core volume (V) × Hardness at midpoint 727 897 692 837 572 437 between core surface and center: Core vh Inner envelope Material No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 layer Thickness (mm) 2.2 2.2 2.2 2.2 2.2 2.9 Volume (cm³) 7.7 7.5 7.5 7.3 7.3 8.0 Material hardness (Shore C) 72 72 72 72 72 72 Material hardness (Shore D) 47 47 47 47 47 47 Inner envelope Diameter (mm) 35.20 35.23 34.73 34.75 34.70 32.49 layer-encased Weight (g) 28.1 28.1 27.1 27.2 27.1 23.3 sphere Deflection (mm) 6.8 5.7 6.8 5.7 7.9 7.0 Surface hardness (Shore C) 82.0 82.2 82.2 82.2 81.1 82.5 Shore hardness (Shore D) 54.3 54.5 54.5 54.5 53.6 54.7 Inner envelope layer volume × 630 620 614 602 593 657 Shore C surface hardness of inner envelope layer-encased sphere: IE vh Outer envelope Material No. 2 No. 2 No. 1 No. 1 No. 1 No. 2 layer Thickness (mm) 1.8 1.8 1.8 1.8 1.8 2.9 Volume (cm³) 7.7 7.6 7.5 7.5 7.6 11.3 Material hardness (Shore C) 82 82 72 72 72 79 Material hardness (Shore D) 51 51 47 47 47 52 Total thickness of envelope layer (mm) 4.0 4.0 4.0 4.0 4.0 5.8 Outer envelope Diameter (mm) 38.78 38.77 38.32 38.32 38.32 38.23 layer-encased Weight (g) 35.4 35.4 34.3 34.4 34.3 34.0 sphere Deflection (mm) 5.8 4.9 5.9 5.1 6.8 5.1 Surface hardness (Shore C) 88.5 88.0 82.2 82.2 82.5 88.3 Surface hardness (Shore D) 59.2 58.9 54.5 54.5 54.7 59.1 Outer envelope layer surface hardness − 45 35 38 29 44 46 Core center hardness (Shore C) Outer envelope layer surface hardness − 32 23 26 17 36 37 Core surface hardness (Shore D) Outer envelope layer volume × 681 671 618 616 624 998 Surface hardness of outer envelope layer-encased sphere: OE vh Comparative Example 3 4 5 6 7 8 Core Diameter (mm) 26.89 35.06 35.07 35.04 30.36 29.59 Weight (g) 16.0 27.7 27.6 27.8 20.3 17.5 Volume V (cm³) 10.2 22.6 22.6 22.5 14.6 13.6 Deflection (mm) 5.3 3.5 3.3 3.6 6.3 6.3 Shore C hardness at surface: Cs 70.1 82.9 82.9 80.2 65.3 64.6 Hardness at position 75% of 61.5 77.5 76.9 77.6 60.2 60.0 core radius from center: C75 Hardness at position 50% of 60.5 72.9 72.4 73.6 57.3 57.1 core radius from center: C50 Hardness at position 25% of 57.0 70.3 70.3 69.0 55.1 55.0 core radius from center: C25 Shore C hardness at center: Cc 56.6 63.6 64.0 64.4 53.3 53.3 Shore C hardness difference 13.5 19.3 18.9 15.8 12.0 11.3 between surface and center (Cs − Cc) (Cs − C75)/(C50 − Cc) 2.2 0.6 0.7 0.3 1.3 1.2 (Cs − C50)/(C25 − Cc) 24.0 1.5 1.7 1.4 4.4 4.4 (Cs − C50)/(C50 − Cc) 2.5 1.1 1.2 0.7 2.0 2.0 Core volume (V) × Hardness at midpoint 617 1648 1636 1656 837 777 between core surface and center: Core vh Inner envelope Material No. 1 No. 1 No. 1 layer Thickness (mm) 2.8 2.2 2.2 Volume (cm³) 7.8 7.3 7.0 Material hardness (Shore C) 72 72 72 Material hardness (Shore D) 47 47 47 Inner envelope Diameter (mm) 32.51 34.75 33.98 layer-encased Weight (g) 23.4 27.2 25.4 sphere Deflection (mm) 4.9 5.7 5.7 Surface hardness (Shore C) 83.4 82.2 82.2 Shore hardness (Shore D) 55.3 54.5 54.5 Inner envelope layer volume × 651 602 574 Shore C surface hardness of inner envelope layer-encased sphere: IE vh Outer envelope Material No. 2 No. 3 No. 1 No. 3 No. 1 No. 1 layer Thickness (mm) 2.9 1.6 1.6 1.6 1.8 1.8 Volume (cm³) 11.3 6.9 7.0 6.8 7.5 7.2 Material hardness (Shore C) 82 80 82 80 72 72 Material hardness (Shore D) 51 50 51 50 47 47 Total thickness of envelope layer (mm) 5.7 1.6 1.6 1.6 4.0 4.0 Outer envelope Diameter (mm) 38.24 38.32 38.36 38.28 38.32 37.55 layer-encased Weight (g) 34.2 34.2 34.2 34.2 34.4 32.3 sphere Deflection (mm) 4.0 3.2 3.2 3.4 5.1 5.1 Surface hardness (Shore C) 88.1 85.9 82.9 85.9 82.4 82.4 Surface hardness (Shore D) 59.0 57.3 55.0 57.3 54.6 54.6 Outer envelope layer surface hardness − 32 22 19 22 29 29 Core center hardness (Shore C) Outer envelope layer surface hardness − 18 3 0 6 17 18 Core surface hardness (Shore D) Outer envelope layer volume × 993 593 578 588 617 592 Surface hardness of outer envelope layer-encased sphere: OE vh

TABLE 6 Example Comparative Example 1 2 3 4 1 2 Intermediate Material No. 4 No. 4 No. 4 No. 4 No. 4 No. 4 layer Thickness (mm) 1.16 1.16 1.18 1.17 1.16 1.22 Material hardness (Shore C) 95 95 95 95 95 95 Material hardness (Shore D) 64 64 64 64 64 64 Intermediate Diameter (mm) 41.10 41.10 40.69 40.67 40.64 40.67 layer-encased Weight (g) 40.9 40.9 39.8 39.8 39.8 39.7 sphere Deflection (mm) 4.3 3.8 4.4 4.0 4.8 3.7 Surface hardness (Shore C) 98 98 98 98 98 98 Surface hardness (Shore D) 70 70 70 70 70 70 Intermediate layer surface hardness − 9 10 15 15 15 9 Outer envelope layer surface hardness (Shore C) Intermediate layer surface hardness − 54 44 54 44 59 56 Core center hardness (Shore C) Cover Material No. 6 No. 6 No. 6 No. 6 No. 6 No. 6 Thickness (mm) 0.80 0.80 0.99 1.00 1.00 0.99 Material hardness (Shore C) 76 76 76 76 76 76 Material hardness (Shore D) 50 50 50 50 50 50 Coating Material C C C C C C layer Material hardness (Shore C) 63 63 63 63 63 63 Ball Diameter (mm) 42.70 42.70 42.67 42.67 42.65 42.65 Weight (g) 45.5 45.5 45.3 45.5 45.5 45.3 Deflection (mm) 3.8 3.4 3.9 3.6 4.2 3.3 Initial velocity (m/s) 77.3 77.5 77.1 77.2 76.8 77.4 Surface hardness (Shore C) 89 89 87 87 87 87 Surface hardness (Shore D) 62 62 61 61 61 61 Total thickness of envelope layer/(Cover 2.1 2.0 1.9 1.8 1.8 2.6 thickness + Intermediate layer thickness) Core diameter/Ball diameter 0.719 0.722 0.709 0.711 0.711 0.627 Intermediate layer surface hardness − 9 9 11 11 11 11 Ball surface hardness (Shore C) Core deflection − Ball deflection: 3.8 2.8 3.6 2.6 4.6 4.7 C − B (mm) Core deflection/Ball deflection: C/B 2.0 1.8 1.9 1.7 2.1 2.4 (OE vh + IE vh)/Core vh 1.8 1.4 1.8 1.5 2.1 3.8 Comparative Example 3 4 5 6 7 8 Intermediate Material No. 4 No. 4 No. 4 No. 4 No. 5 No. 4 layer Thickness (mm) 1.21 1.21 1.16 1.18 1.17 1.17 Material hardness (Shore C) 95 95 95 95 84 95 Material hardness (Shore D) 64 64 64 64 56 64 Intermediate Diameter (mm) 40.66 40.74 40.68 40.64 40.67 39.90 layer-encased Weight (g) 39.8 39.7 39.6 39.7 39.8 37.6 sphere Deflection (mm) 3.2 2.7 2.9 2.9 4.0 4.0 Surface hardness (Shore C) 98 98 98 98 92 98 Surface hardness (Shore D) 70 70 70 70 62 70 Intermediate layer surface hardness − 9 12 15 12 10 15 Outer envelope layer surface hardness (Shore C) Intermediate layer surface hardness − 41 34 34 33 39 44 Core center hardness (Shore C) Cover Material No. 6 No. 6 No. 6 No. 6 No. 7 No. 6 Thickness (mm) 1.00 0.98 1.00 1.00 1.00 1.40 Material hardness (Shore C) 76 76 76 76 86 76 Material hardness (Shore D) 50 50 50 50 57 50 Coating Material C C C C C C layer Material hardness (Shore C) 63 63 63 63 63 63 Ball Diameter (mm) 42.66 42.70 42.68 42.65 42.67 42.70 Weight (g) 45.5 45.3 45.3 45.4 45.5 45.5 Deflection (mm) 2.9 2.6 2.7 2.7 3.6 3.4 Initial velocity (m/s) 77.4 77.2 77.6 77.2 76.8 76.7 Surface hardness (Shore C) 87 87 87 87 94 86 Surface hardness (Shore D) 61 61 61 61 64 57 Total thickness of envelope layer/(Cover 2.6 0.7 0.8 0.7 1.8 1.5 thickness + Intermediate layer thickness) Core diameter/Ball diameter 0.630 0.821 0.822 0.822 0.711 0.693 Intermediate layer surface hardness − 11 11 11 11 −2 12 Ball surface hardness (Shore C) Core deflection − Ball deflection: 2.4 0.9 0.6 0.9 2.7 2.9 C − B (mm) Core deflection/Ball deflection: C/B 1.8 1.4 1.2 1.3 1.7 1.8 (OE vh + IE vh)/Core vh 2.7 0.4 0.4 0.4 1.5 1.5

The flight performance (I #6), spin rate on approach shots, feel at impact and durability on repeated impact of each golf ball are evaluated by the following methods. The results are shown in Table 7.

Flight Performance (I #6)

A number six iron (I #6) is mounted on a golf swing robot and the distance traveled by the ball when struck at a head speed of 44 m/s was measured and rated according to the criteria shown below. The club used is the TourB X-CB (I #6) manufactured by Bridgestone Sports Co., Ltd. In addition, the spin rate is measured with a launch monitor immediately after the ball is similarly struck.

Rating Criteria

-   -   Good: Total distance is 179.0 m or more     -   NG: Total distance is less than 179.0 m

Evaluation of Spin Rate on Approach Shots

A sand wedge (SW) is mounted on a golf swing robot and the amount of spin by the ball when struck at a head speed of 20 m/s was rated according to the criteria shown below. A launch monitor is used to measure the spin rate immediately after the ball was struck. The sand wedge is the TourB XW-1 (SW) manufactured by Bridgestone Sports Co., Ltd.

Rating Criteria:

-   -   Good: Spin rate is 5,900 rpm or more     -   NG: Spin rate is less than 5,900 rpm

Feel

The feel of the ball when struck on a full shot with an iron (I #6) by amateur golfers having a handicap of 15 to 25 is rated according to the criteria shown below.

Rating Criteria:

-   -   Good: Fifteen or more out of 20 golfers rated the ball as having         a very soft and good feel     -   Fair: At least 10 and up to 14 out of 20 golfers rated the ball         as having a soft and good feel     -   NG: Nine or fewer out of 20 golfers rated the ball as having a         soft and good feel

Durability to Cracking on Repeated Impact

A driver (W #1) is mounted on a golf swing robot, N=10 sample balls are repeatedly struck at a head speed of 45 m/s, and the durability of the balls is evaluated according to the criteria shown below.

Evaluation Criteria:

Using ten balls, the number of shots required for each ball to begin cracking is counted. Of the ten balls, the three balls having the lowest number of shots are selected, and the average number of shots for these three balls is treated as the “number of shots required for cracking.” Durability indices for the balls in the respective Examples are calculated relative to an arbitrary value of 100 for the number of shots required for the ball in Example 3 to crack.

-   -   Good: Index is 90 or more     -   NG: Index is less than 90

TABLE 7 Example Comparative Example 1 2 3 4 1 2 3 4 5 6 7 8 Flight I#6 Spin 5,206 5,351 5,061 5,334 5,125 5,555 5,825 6,533 6,546 6,363 5,579 5,504 HS: rate 44 (rpm) m/s Total 183.3 181.8 182.6 179.5 180.2 178.4 181.8 173.6 176.9 175.9 179 178.3 distance (m) Rating good good good good good NG good NG NG NG good NG Approach HS: Spin 5,921 5,973 5,996 6,116 6,044 6,179 6,339 6,381 6,386 6,361 5,864 6,320 shots 20 rate m/s (rpm) Rating good good good good good good good good good good NG good Feel at impact Rating good good good good good fair NG NG NG NG good good Durability to Rating good good good good NG NG good good good good good good repeated impact

As demonstrated by the results in Table 7, the golf balls of Comparative Examples 1 to 8 are inferior in the following respects to the golf balls according to the present invention obtained in Examples 1 to 4.

In Comparative Example 1, the value of the core volume V (cm³) multiplied by C50 (Core vh) is smaller than 630 and the value of (Cs−C75)/(C50−Cc) is larger than 2.3. As a result, the durability to cracking on repeated impact is poor.

In Comparative Example 2, the value of the core volume V (cm³) multiplied by C50 (Core vh) is smaller than 630, the value of (Cs−C75)/(C50−Cc) is larger than 2.3 and the value of (core deflection)/(ball deflection) is larger than 2.3. As a result, the durability to cracking on repeated impact is poor, in addition to which the distance traveled by the ball on shots with an iron is inferior and the feel at impact is less than satisfactory.

In Comparative Example 3, the value of the core volume V (cm³) multiplied by C50 (Core vh) is smaller than 630. As a result, a good feel at impact is not obtained.

In Comparative Example 4, the ball deflection is smaller than 2.8 mm and the (ball deflection)/(core deflection) value is smaller than 1.6. In addition, the value of the core volume V (cm³) multiplied by C50 (Core vh) is larger than 1,000 and the value of (Cs−C75)/(C50−Cc) is smaller than 1.0. As a result, the distance traveled by the ball on shots with an iron is poor and a good feel is not obtained.

In Comparative Example 5, the ball deflection is smaller than 2.8 mm and the (ball deflection)/(core deflection) value is smaller than 1.6. In addition, the value of the core volume V (cm³) multiplied by C50 (Core vh) is larger than 1,000 and the value of (Cs−C75)/(C50−Cc) is smaller than 1.0. As a result, the distance traveled by the ball on shots with an iron is poor and a good feel is not obtained.

In Comparative Example 6, the ball deflection is smaller than 2.8 mm and the (ball deflection)/(core deflection) value is smaller than 1.6. In addition, the value of the core volume V (cm³) multiplied by C50 (Core vh) is larger than 1,000 and the value of (Cs−C75)/(C50−Cc) is smaller than 1.0. As a result, the distance traveled by the ball on shots with an iron is poor and a good feel is not obtained.

In Comparative Example 7, the ball surface hardness is higher than the intermediate layer surface hardness. As a result, the spin rate of the ball on shots with an iron rises and a good distance is not obtained. In addition, the spin rate on approach shots is low.

In Comparative Example 8, the thickness of the cover (outermost layer) is larger than 1.0 mm. As a result, the spin rate on shots with an iron rises, the initial velocity of the hall when struck is low, and the distance is inferior.

Japanese Patent Application No. 2021-191634 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A multi-piece solid golf ball comprising a core, an envelope layer, an intermediate layer and a cover, wherein the core is formed of a rubber composition as one or more layer; the envelope layer is formed of a resin material as one or more layer; the intermediate layer is formed of a resin material as one layer; the cover is formed of a resin material as one layer having a thickness of not more than 1.0 mm; the Shore C hardness at a surface of the core, the Shore C hardness at a surface of the sphere obtained by encasing the core with the envelope layer (envelope layer-encased sphere), the Shore C hardness at a surface of the sphere obtained by encasing the envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) and the Shore C hardness at a surface of the ball together satisfy the conditions (Shore C hardness at surface of envelope layer-encased sphere)>(Shore C hardness at core surface) and (Shore C hardness at surface of intermediate layer-encased sphere)>(Shore C hardness at ball surface); letting C be the deflection of the core in millimeters when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and B be the deflection of the ball in millimeters when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), B≥2.8 and 1.6≤C/B≤2.3; the core has an internal hardness which satisfies the condition: 1.0≤(Cs−C75)/(C50−Cc)≤2.3, where Cc is the Shore C hardness at the core center, Cs is the Shore C hardness at the core surface, C75 is the Shore C hardness at a position 75% of the core radius from the core center toward the core surface and C50 is the Shore C hardness at a position 50% of the core radius from the core center to the core surface; and the core satisfies the condition: 630≤Core vh≤1,000, where Core vh is the product of the core volume V (cm³) multiplied by C50.
 2. The golf ball of claim 1, wherein the interior hardness of the core satisfies the condition: 3.0≤(Cs−C50)/(C25−Cc)≤20.0, where C25 is the Shore C hardness at a position 25% of the core radius from the core center toward the core surface.
 3. The golf ball of claim 1, wherein the interior hardness of the core satisfies the condition: (Cs−C50)/(C50−Cc)≥1.1.
 4. The golf ball of claim 1, wherein the Shore C hardness at the surface of the envelope layer-encased sphere and the Shore C hardness at the surface of the intermediate layer-encased sphere together satisfy the condition: (Shore C hardness at surface of intermediate layer-encased sphere)>(Shore C hardness at surface of envelope layer-encased sphere).
 5. The golf ball of claim 1, wherein the layers have respective thicknesses which together satisfy the conditions: (cover thickness)<(intermediate layer thickness)≤(total thickness of envelope layer).
 6. The golf ball of claim 1, wherein the layers have respective thicknesses which together satisfy the condition: (total thickness of envelope layer)/(cover thickness+intermediate layer thickness)≥1.0.
 7. The golf ball of claim 1, wherein the envelope layer is formed of at least two layers that include an inner envelope layer and an outer envelope layer.
 8. The golf ball of claim 7, wherein the Shore C hardnesses at the surfaces of the respective layers satisfy the following conditions: (Shore C hardness at ball surface)<(Shore C hardness at surface of intermediate layer-encased sphere)>(Shore C hardness at surface of outer envelope layer-encased sphere)≥(Shore C hardness at surface of inner envelope layer-encased sphere)>(Shore C hardness at core surface).
 9. The golf ball of claim 1, wherein the core has a diameter and the ball has a diameter which together satisfy the condition: 0.65≤(core diameter)/(ball diameter)≤0.80. 